Figure 1. RBOF spent fuel storage basin.
Figure 2. Corrosion Product on Underwater stored Mk 31 Fuel Assemblies
Figure 3. Nodular corrosion product on underwater-stored Mk 22 fuel assemblies.
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Research and test reactors around the world are currently returning spent fuel
originally enriched in the United States back to the U. S. In May 2006,
operators of the reactors will cease to be eligible to return their spent fuel
and will have to find national or regional solutions for continued storage,
if they want to continue operations of their research reactors. The Savannah
River Site (SRS) has looked at a number of options like dry storage,
melt-dilute, or continued wet storage for fuel currently stored at SRS.
This paper reviews the highlights of the wet basin studies conducted at SRS since 1992. Based on an understanding of the important factors affecting the corrosion of aluminum-clad spent fuel, criteria are presented for the corrosion protection of this fuel in extended water storage. With optimum water quality, aluminum-clad spent fuel can be stored safely and with minimum corrosion for times exceeding 25 years.
Research and test reactors around the world are currently returning spent fuel
originally enriched in the United States back to the U. S. In May 2006,
operators of the reactors will cease to be eligible to return their spent fuel
and will have to find national or regional solutions for continued storage,
if they want to continue operations of their research reactors. For research
reactors with fuel enriched in the former Soviet Union and other countries,
some of their spent fuel facilities are filled to near capacity while others
have reached their design lifetimes.
At Westinghouse Savannah River Site (SRS) where light-water filled basins are
currently storing remaining production and research reactor aluminum-clad spent
fuel, a number of studies have been undertaken over the last few years. These
studies have been directed toward ways to extend wet basin storage and at
alternate technologies such as dry storage and a process for consolidation and
lowering the enrichment of these aluminum-clad fuels known as the melt-dilute
process.
This paper reviews highlights of the wet basin studies and presents criteria for corrosion protection for extended wet storage. This information could prove beneficial for research and test reactor operators formulating spent fuel management plans for 2006 and beyond.
At the Savannah River Site, aluminum-clad spent fuel is currently being stored
in water-filled basins associated with the L-Reactor, the K-Reactor, and the
stand alone Receiving Basin for Off-Site Fuels (RBOF). The reactor basins have
been used in the past primarily for storing aluminum-clad fuel and target
materials while awaiting processing and stabilization in the Separations
facilities. The RBOF basin, as shown in Figure 1, was built in the mid-1960's
to store spent fuel received from research and test reactors around the
world.
When processing of spent fuel was suspended at SRS and other reactors within
the DOE Complex in late 1989, over 200 Metric Tons Heavy Metal (MTHM) of
aluminum-clad spent fuel were stored in the basins at SRS.1 Fuel
that normally stayed in the basins for 12-18 months accumulated several years
of exposure in less than optimum water conditions. As a result, pitting
corrosion of the aluminum-clad caused breach of this protective cladding.
2 Corrosion on these fuel and target materials in the early 1990's is
shown in Figure 2 and Figure 3.
In order to develop a fundamental understanding of
the corrosion problems with the aluminum-clad spent fuel in the SRS wet storage
basins in early 1992, a corrosion surveillance program was implemented. The
program consisted of in-basin tests using
corrosion surveillance coupons and laboratory electrochemical tests.
Information obtained from these programs provided the basis for recommendations
for components to plant equipment and basin operations to mitigate corrosion of
the spent fuel.
These tests were initiated in early 1992 in the reactor storage basins and in
RBOF in 1994. Surveillance coupons were cut from actual unirradiated Mark 22
fuel assemblies and pre-oxidized to give a 1-micron thick Boehmite phase oxide
on the surface. The corrosion racks, each consisting of 6 nested coupons as
shown in figure 4 were immersed into storage basins and removed at periodic
intervals and examined for corrosion. Table 1 shows results for these
withdrawals through 1999.3-5
Results from the early withdrawals in K-Reactor Basin showed that pitting corrosion was aggressive in water that had 180 µS/cm and 8 ppm chloride. The 30-mil aluminum cladding was breached in as little as 45 days. With further deionization of the water, no pitting was observed on coupons in L and K-basins when the water conductivity was lowered to the 100 -125 µS/cm range. No pitting corrosion has ever been observed on corrosion coupons in RBOF. Water conductivity in this basin has been maintained routinely in the 1-3 µS/cm range. Based on the understanding developed from these field tests and laboratory corrosion tests, the factors believed to have placed the most important rate in the corrosion of the aluminum-clad spent fuel in the reactor basins at SRS were:
A concentrated basin cleanup program was initiated in 1994. Based on initial observations and the understanding about what was driving the corrosion process, recommendations were made to improve the water quality. The following steps were taken to cleanup the basins:
After a three-year program to cleanup the basins, the water quality is outstanding in reactor basins. The RBOF facility with water conductivity maintained in the 1-3 µS/cm range has never experienced corrosion problems. The water conductivity of L and K- Basins is now maintained routinely near 1 µS/cm with the aggressive ions like chloride are in the ppb range. As a result, no new pitting corrosion has been observed on corrosion surveillance coupons since the tests were initiated there in 1994.
Most of the fuel used in research reactors of eastern and western countries is
fabricated with a uranium-aluminum alloy core protected by an aluminum alloy
cladding material. A small percentage of the fuel is clad with stainless steel,
zirconium, or other alloys. Fuel is regarded as spent nuclear fuel (SNF)
regardless of burnup when it is discharged from the reactor core for the final
time. It is then normally placed in pools for cooling and interim storage
until a final disposition is made. Some of this aluminum-clad spent fuel has
been in water storage in these countries for more than 40 years and remains in
pristine condition while other fuel has been severely degraded by pitting
corrosion. Pitting corrosion of the fuel can lead to breach of the cladding
material and release of radioactivity to the storage basin.
Based on a fundamental understanding of the corrosion processes involved in the
corrosion of aluminum-clad spent fuel at the SRS and other fuel storage basins
around the United States, a criteria has been developed in this paper to
optimize corrosion protection for aluminum-clad spent nuclear fuel in wet
storage pools. The criteria include recommended water chemistry, operational
practices, and other basin management techniques for extended interim storage.
The principles applied to obtain optimum water quality will also result in low
corrosion rates for the other cladding alloys used with research reactor
fuel.
Research reactor spent fuel is being stored in water filled basins around the world under a wide variety of storage conditions. Many of the pools have water purification equipment that maintains the water at high purity levels. In these basins, aluminum-clad fuels have been stored for 25-35 years without corrosion problems. Other storage pools are small and do not have high quality water. For these pools, corrosion of aluminum clad SNF has been a concern. As a part of the U.S. Department of Energy's decision to return foreign research reactor spent fuel to the United States, over 1700 aluminum-clad assemblies have been inspected for corrosion and mechanical damage.6 The condition of assemblies has ranged from pristine, with no visible corrosion, to severe and localized nodular corrosion with pitting. Approximately 7% of the assemblies inspected showed pitting, which was judged to be through the aluminum clad into the core of the fuel.
The corrosion of aluminum-clad spent nuclear fuel is dependent on a number of
interrelated factors. Many of the metallurgical factors are already inherent in
the spent fuel when the reactor operator receives the fuel from the fuel
fabricator for irradiation. Factors such as alloy composition, heat treatment,
microstructure; nature and thickness of the protective oxide coating,
inclusions and impurities in the alloy, and cold work play a role in the
corrosion process, but are not controllable during wet storage. The
recommended guidelines presented in this paper apply to the environmental and
service related factors that are normally controllable and can be used to
optimize the corrosion protection of the aluminum-clad fuel during interim wet
storage.
These guidelines are designed to prevent breach of this clad and subsequent corrosion of the fuel core. Most of the research reactor fuel is fabricated from uranium-aluminum alloy and this type of fuel exhibits a corrosion behavior similar to aluminum. Therefore, implementation of these guidelines should also minimize corrosion of the fuel core. The corrosion of a metallic uranium core is a much more rapid chemical reaction than that for a uranium-aluminum alloy fuel, but would still be reduced by implementation of these recommendations to protect the aluminum clad.
Water Chemistry. Maintaining high quality water chemistry
in the fuel storage pool is the single most important factor in controlling
corrosion of aluminum-clad spent fuel assemblies and other aluminum components
stored in the pool. Treatment and purification of the water in the pool and
any make-up water through filters and using ion exchange resins is essential to
achieving optimum storage performance. The guidelines presented here provide
the recommended water parameters to minimize pitting and other forms of
corrosion on aluminum-clad spent fuel during extended interim wet storage:
Conductivity. The conductivity of the water in the fuel storage
basin shall be maintained as low as achievable and in the range of 1-3
µS/cm for optimum corrosion protection. This level may be
difficult to achieve in unlined pools. A level of 3-10 µS/cm may yield
satisfactory results provided impurities like chloride ions are in low
concentrations. There is some evidence that pitting may be suppressed below 50
µS/cm depending on other favorable conditions. Values near 200
µS/cm are known to be aggressive to pitting of aluminum.
pH. The pH shall be maintained in reactor pools in a range of
5.5 to 6.5. This pH level will minimize uniform corrosion. Pitting corrosion
is relatively independent over this pH range. Tight control of the pH is
essential for reactors that have the core and fuel storage basin sharing the
same cooling water. For away from reactor storage pools, a somewhat wider range
of 5.0-8.0 may be permissible. Irradiation is known to reduce the stability
range of the protective oxide and can result in extensive turbidity from
precipitation of aluminum hydroxide from the water.
Chloride (Cl). The chloride ion content of the water shall be
maintained as low as achievable and less than 1 ppm for optimum corrosion
protection. This level is generally achievable if water
conductivity is maintained in the 1-3 µS/cm range. Chloride ions
break down the passive film on aluminum and promote metal dissolution.
Sulfates (SO4). The total sulfate ion content
of the water shall be maintained at less than 1 ppm for optimum corrosion
protection. However, for unlined pools where chemistry is hard to control, a
level at or below 10 ppm should give satisfactory protection. An increase in
sulfate concentration results in a decrease in thickness of the protective
oxide film with a corresponding increase in the susceptibility to pitting
corrosion.
Heavy Metals. The concentration of copper (Cu), mercury
(Hg), silver (Ag), and other heavy metal ions shall be maintained at or
below 0.02 ppm. Heavy metal ions are extremely aggressive to the pitting
corrosion of aluminum as they can plate out readily forming strong galvanic
cells. These ions have strong synergistic reactions with chloride,
bicarbonate, and calcium ions. Reduced metals in the basin sludge or
particulate in solutions can form galvanic cells leading to localized corrosion
when the particles are in contact with the aluminum.
Other Impurities. Impurity ions such as iron (Fe), aluminum
(Al), nitrates (NO3), nitrite (NO2) shall be maintained
at levels as low as possible. Normal deionization of the water in the storage
pool to 1-3 µS/cm should keep these impurities to the 1 ppm or
lower level. Any addition of cations or anions to the water increases the
water conductivity and offers less resistance to the flow of corrosion current
from the aluminum-clad.
Hardness. Maintain carbonate hardness of the water at 60 ppm or
less when possible. Carbonate (CO3-) and
bicarbonate ions (HCO3-) and can react synergistically
with chloride and copper and lead to intensive pitting of aluminum.
Soft water as defined by a carbonate content of 60 ppm or less is less
aggressive to aluminum corrosion than hard water at a carbonate content of 60
ppm or greater. Continuous deionization of the basin water softens the water
as it removes calcium carbonate and other ions contributing to the hardness.
Temperature. The water temperature shall be maintained at 40
0C or below. The rate of pitting at pitting at 40 0C has
been found to be 5 times the pitting rate at 25 0C. The density and
probability of pitting has been found to increase with temperature. The
corrosion rate of uranium metal increases dramatically with increasing
temperature.
Radiation Effects. Gamma radiation from irradiated fuel assemblies, cobalt-60 or radioactive cesium sources can have some effect on materials stored in fuel storage pools. The gamma fluxes, however, have little effect on the properties of the metal cladding and the radiation field does not seem to promote any significant increases in corrosion of the metals in wet storage. Gamma fields can deteriorate components subjected to radiolytic decomposition like neutron absorbers that include organic materials and rack configurations that trap water that subsequently forms gas pressure from decomposition.
Water Circulation. Avoid stagnant areas of water in the fuel
storage basin. Ensure that the water is circulated to provide movement over
the stored fuel assemblies. As an example, a linear flow rate over the
aluminum surface as little as 2.4 meters per minute has been shown to
suppress pitting on some aluminum alloys.
Sludge Removal. Do not allow sludge to accumulate in the water
of the fuel storage basin. Remove on a periodic basis by vacuuming or other
methods. This material can concentrate chlorides, heavy metals, etc. and
deposit on fuel assemblies initiating pitting of the aluminum clad.
Filtration. Mechanical filters or resin beds shall be used to
control suspended solids or particulate material in the basin water before it
turns into sludge. Deionization of the water helps to accomplish this.
Skimmer System. Debris and other species floating on the
water surface of fuel storage pools shall be removed by a skimmer system or
other means. This material can settle on surfaces of fuel cladding and cause
pitting corrosion.
Crevices. Avoid crevices between the aluminum-clad assemblies
and the fuel storage racks or hangers. Low pH water conditions, concentration
of chloride ions, and oxygen concentration cells in these crevices can lead to
accelerated corrosion of the cladding.
Galvanic Couples. Avoid contact between aluminum-clad fuel
assemblies and dissimilar metal storage racks or hangers. Use aluminum storage
racks or provide non-conducting insulators whenever possible. Aluminum-clad
fuel coupled to stainless steel racks or hangers will accelerate pitting
corrosion of the aluminum.
Handling of Spent Fuel. Avoid handling fuel assemblies with
sharp edged tools as scratches in the oxide coating of irradiated fuel serve as
pitting initiation sites when in the storage basin. Minimize, as much as
possible, mechanical damage and surface scratches on fuel element surfaces
during discharge from the reactor core and during subsequent fuel handling and
storage operations.
Microbiological Activity. Do not add chemicals containing
chlorides or other halogens such as sodium hypochlorite (NaOCl) to the water in
storage basins for control of algae, bacteria, or turbidity without first
testing for compatibility with the fuel, basin lining, or other basin
components. The chloride in many of the chemicals will destroy the passive
film on aluminum and cause aggressive pitting corrosion.
Biofilm Formation at Air/Water Interface. The bath tub ring
often formed at the air/water interface around the sides of the basin is likely
a biofilm of microbial activity. This film acts like a trap and is known for
concentrating cesium and other radioactive isotopes contained in the basin
water. This biofilm should be removed mechanically by wet brushing using water
to hold down any airborne activity. A 35% solution (200mL in 700 gal) of
hydrogen peroxide (H2O2) has proven effective in killing
microbial activity and could be used to assist this removal without corrosive
attack on aluminum alloys.
Basin Lighting Conditions. The lighting conditions shall be
maintained as low as practical in and around the basin area. High levels of
lighting promote microbiological growth activity in the water. Ultraviolet
lighting can be used to suppress microbiological activity associated with
filters, etc. However, sidestream ultraviolet light systems are used primarily
for planktonic activity and are not effective on sessile colonies.
Make-up Water. Additions of water to the fuel storage
pool should be of a quality equal to or better than the existing pool water.
Deionized water shall be used whenever possible.
Radionuclide Activity in the Basin Water. Radionuclide activity
in the basin water leached from the spent fuel shall be monitored and
controlled to levels deemed to be safe for personnel working in the surrounding
area. Continuous deionization of the water removes alpha and beta-gamma
radioactivity from the water. Fission products such as Cesium-137 and other
radionuclides may be found in the water from the storage of failed spent fuel
elements or breach of clad. Special materials such as zeolite can be used in
the ion exchange type purification system to specifically target these
radionuclides for removal.
Water Sampling Plan. Maintaining water purity levels to the
strict guidelines presented here is vital for successful operation of a wet
storage fuel facility. Basin water quality is monitored through sampling, and
trending sampling results. All the major water parameters such as pH,
conductivity, and chlorides shall be measured on a periodic basis consistent
with good basin management practices. Weekly monitoring is recommended, but
this interval can be established by the basin operator and is dependent on pool
conditions. Other impurity ions such as sulfates, nitrates, nitrites, copper,
mercury, iron, and aluminum shall be measured quarterly as a minimum.
Temperature shall be monitored daily. The alpha and beta-gamma radioactivity
measurements in basins storing spent fuel shall be made on a frequency basis
established by individual requirements. An increase in radioactivity is an
indicator of corrosion. Permanent records shall be kept and analytical
results trended.
A baseline evaluation of microbiological activity in the basin shall be made
from a sampling of the basin water. Analysis shall include counts of
hetrotrophic, acid producing, anaerobic, and sulfate reducing bacteria. This
baseline can be use to compare with analysis of bacteria at some later time.
Corrosion Surveillance Program. A long-term corrosion
surveillance program shall be implemented in the fuel storage pool to monitor
the aggressiveness of the basin water toward the corrosion of aluminum alloys.
Details for specific site programs can be tailored for the individual sites,
but should contain the generic elements described in this paper. A typical
program has been implemented in a number of countries with research reactor
fuel.7 Corrosion racks with coupons manufactured from aluminum
alloys typical of the spent fuel cladding shall be immersed in the water near
the stored aluminum-clad spent fuel. Standard corrosion coupons, either round
or rectangular shaped shall be used. Multiple coupons representing individual
aluminum alloys, crevice corrosion coupons, and galvanic coupons shall comprise
each rack. The coupons shall not be pre-oxidized other than air-formed oxide
normally found on the surface. This insures some conservatism.
A schedule for withdrawal of these coupons shall be formulated based on the
length of time fuel is expected to be in the basin and the total number of
corrosion racks available for testing. As a minimum there should be enough
racks available to obtain data after 6 months, one year, and two years to
provide an early indication of the aggressiveness of the storage environment on
the aluminum coupons. Depending on the expected storage life of the fuel, the
surveillance program should continue. For a twenty-year program, withdrawals
could be extended to 5 years, ten years, and twenty years exposure time. If the
water conditions are more aggressive, additional racks may be required and more
frequent intervals of withdrawal may be necessary. Ideally, however, enough
racks should be purchased to permit at least one withdrawal each year.
Metallographic evaluation of the corrosion coupons shall include pitting
density and maximum pit depth. Knowing pitting depth and exposure time, the
pitting rate may be calculated. Video and still color photography shall be
used to document the withdrawals. Data from the water analyses taken during
the exposure periods should be correlated with the corrosion observations on
the coupons to explain the test results.
Records Management Program. Data from the individual basin water chemistry and corrosion surveillance programs shall be maintained at the individual sites.
The storage of aluminum-clad spent nuclear fuel in less than optimum quality
water can result in aggressive pitting corrosion. This corrosion was a concern
at SRS in the early 1990's, but an aggressive Basin Management program resulted
in significantly improved storage conditions and no new pitting has been seen
in the reactor basins since 1994.
The knowledge learned from these basin cleanup activities and the corrosion surveillance program, underway since 1992, have provided a basic understanding of the important factors affecting the corrosion of aluminum-clad fuel. The criteria for corrosion protection of this fuel have been presented in this paper. Even though there are a number of important factors affecting this corrosion and some act synergistically, the key to extended storage is based on improved water quality conditions. Improved water quality through continuous deionization results in low water conductivity and low impurity concentrations of important corrosion causing species like chloride ions. Aluminum-clad spent fuel can be stored safely and with minimum corrosion for times exceeding 25 years in water with a conductivity near 1 µS/cm and ppb quantities of impurities.
|
|||||||||
Basin |
Date |
Exposure (Days) |
Maximum Pit Depth (Mils) |
Pit
Density |
Avg. |
|
|
|
Avg. |
|
|
|
1100
8001 |
1100
8001 |
8 |
20 |
15 |
7.5 |
175 |
K |
6-92 |
75 |
13 45 |
0.125 .01 |
|||||
K |
3-92 |
107 |
23 39 |
0.125 .01 |
|||||
K |
6-92 |
182 |
58 27 |
0.125 .01 |
|||||
K |
12-92 |
365 |
100 57 |
0.125 .05 |
|||||
1993-2000 |
|||||||||
K |
3-94 |
65 |
No Pitting |
||||||
K |
7-94 |
181 |
|||||||
K |
2-95 |
403 |
6 |
18 |
9 |
7.3 |
125 | ||
K |
8-95 |
590 |
|||||||
K |
8-97 |
1091 |
|
<0.1 |
<0.1 |
<0.1 |
7.25 |
2.5 | |
L |
11-93 |
61 |
No Pitting |
||||||
L |
3-94 |
127 |
|||||||
L |
7-94 |
241 |
|||||||
L |
2-95 |
336 |
14 |
20 |
2 |
6.6 |
102 | ||
L |
8-95 |
340 |
|||||||
L |
8-97 |
1114 |
|
<0.1 |
<0.1 |
<0.1 |
6.6 |
1.8 | |
P |
11-93 |
61 |
No Pitting |
||||||
P |
3-94 |
127 |
|||||||
P |
7-94 |
241 |
|||||||
P |
2-95 |
336 |
35
mil Pits in 8001 |
10 |
9 |
18 |
7.5 |
160 | |
RBOF |
9-97 |
1222 |
No
Pitting |
0.1 |
6.19 |
1.18 | |||
RBOF |
8-95 |
480 |
No Pitting |
0.2 |
1.5 |
0.5 |
7.4 |
1 | |
RBOF |
4-96 |
723 |
No Pitting |
||||||